Spray guns, as used in vehicle body shops for example, are well known and comprise a reservoir in which a liquid to be dispensed is contained, and a spray nozzle through which the liquid is dispensed, typically under pressure, under the control of a trigger mechanism, see for example FIG. 1. The liquid may be fed from the reservoir under gravity and/or it may be entrained in a stream of pressurised fluid, for example air and water, which is supplied to the gun from an external source.
The spraying of colour match test cards (also known as spray out cards), or small areas of vehicles, is often carried out prior to the painting of a whole vehicle. Test cards, see for example FIG. 3, are typically made from cardboard, metal or plastic and can have a comparison hole, typically 7 mm in diameter, cut through them and an opacity check region. Paint to be sprayed onto the test card is premixed from commercially available tinters, typically mixed in small volume containers such as, for example, plastic or paper cups, and an aliquot is loaded into the spray gun. A sufficient number of coats of the paint, which may be either solvent based or water based, to totally obscure the opacity check region are sprayed onto the card. The card is then baked dry and a number of coats of a lacquer, which is typically solvent based, are applied over the paint. The lacquer is then baked dry and the paint compared to the desired colour through the comparison hole by overlaying the card onto a panel of the vehicle having the desired colour.
Small test aliquots of paint are generally taken from large batches of mixed pigments in order to increase the uniformity of the mixture as any error in mixing the pigments to make the paint mixture is amplified by the use of small weights in mixing, for example, a 5 g error in mixing 50 g of paint is a 10% error whereas the same weight error in mixing 500 g represents only 1% error in pigment mix. Weight measurements have been found to be more accurate than volumetric measurements for mixing purposes. However, if a standard, large volume, paint reservoir is used on the spray gun to spray a test piece and the paint is not a good match a large amount of cleaning of the system is required prior to spraying another test piece with a retinted paint mixture in order to prevent cross-contamination between the two paint mixtures. The cleaning of the system is a time consuming, labour intensive operation, increases the amount of solvents used and can also expose users to noxious solvent vapours. One solution to this problem, the use of a disposable paint reservoir liner, see for example FIG. 2, has been discussed in the PCT Application No. PCT/US98/00796 published under No. WO 98/32539 which is assigned to the Minnesota Mining and Manufacturing Company and the contents of which are incorporated herewith by reference.
The use of standard size beverage cups for mixing also reduces the opportunities for cross-contamination between colours as the cups are readily disposed of after use and a new cup is used for each colour. This can lead to large variations in colour between small volume paint mixes and large volume paint batches which are ostensibly the same colour.
The use of small volume containers, such as for example cups, for the mixing of paints does however have a number of problems associated with it. The containers are typically open to their surroundings which can lead to contamination of the paint, generally by particulates or, less likely, by water. The particulates may block the flow path or spray nozzle of the spray gun thereby preventing the spraying of the test card or vehicle and necessitate a complete strip down, clean and overhaul of the spray gun. The introduction of water into a non-water based paint mixture can lead to the formation of a two-phase system or an emulsion that results in an uneven flow rate due to the differing hydrodynamic properties of the paint mixture and water.
Another problem associated with the use of small, open containers such as cups for colour mixing purposes is that it requires the transfer of the pigments from storage containers to the container in order to achieve the desired shade. This can entail the use of jugs that must be thoroughly cleaned with solvents after each use in order to prevent cross-contamination between pigments. The solvents employed in the cleaning process are often inflammable, noxious and/or poisonous and therefore it is desired to minimise their usage wherever possible both for environmental and health reasons.
Different batches of a colour made to ostensibly the same recipe, from ostensibly the same pigments, will typically vary slightly in coloration. Therefore it is usual practice to mix a large batch of a given colour and make variations to the mixture of this large batch and transfer a small amount of the batch to the spray gun reservoir for colour matching purposes. However, varying the paint mixture in the reservoir to achieve a colour match and to “scale up” these variations to the batch in order to attempt to achieve a colour match in the large batch is not always successful for the reasons discussed hereinbefore.
Another problem of current paint mixing arrangements is that should a customer wish to retain a small amount of the paint, for example, for possible “touching up” of any future scratches to their vehicle, the paint must be transferred to a sealable container for them. This again raises the problems of contamination of the paint by particulates and also the need to dispose of the reservoir after use.
A paint feed system is disclosed in JP 11290728 comprising a pair of piston/cylinder units operable in tandem to provide a continuous supply of paint to a robotic spray gun. The piston of each unit is driven by a servomotor under the control of a controller to connect alternately the units to the paint supply and spray gun. In this way, one unit takes up paint as the piston is retracted and the other unit delivers paint as the piston is advanced. This system is designed for continuous supply of paint of one colour and is not suitable for rapid changing over between colours due to the extensive cleaning of the units and supply lines that would be required. It is also designed for use with a remote spray gun such as carried by a robot arm and is neither intended nor capable of being used with a hand-held spray gun.